ESD protection devices typically operate either in normal mode, wherein a control gate is controlled by a bias voltage to define an active clamp, or operate in snapback mode based on punch-through effect, which occurs when the forward blocking voltage is reached.
In the latter case, the forward blocking voltage depends on the transient biasing dynamics, and can therefore lead to early triggering. Thus, while snapback ESD protection devices are designed to be triggered by an ESD pulse at a voltage magnitude in the ESD protection window, these devices can in fact be inadvertently turned on during normal operation when used in high voltage, fast switching applications. Thus this becomes an issue when dealing with ESD protection of fast switching devices such as fast switching voltage regulators, which during normal operation display switching times in the range of 10 ns-200 ps. Thus they can have switching times that are faster than the ESD rise time (10 ns for HBM pulse).
While NLDMOS SCR devices are highly suited for high voltage node ESD protection, they rely on passive switching due to a triggering voltage that exceeds the forward blocking voltage.